Seat-belt pretensioners are used in motor vehicles to enhance occupant protection of a belted occupant at the start of an accident by retracting the seat-belt to couple the passenger to the vehicle deceleration as soon as possible, Because of the early coupling of the passenger, the restraint force acting on him or her is reduced, in that it is distributed on the longest possible path of forward displacement. In order that the longest possible path of forward displacement is also utilized in full, the seal-belt retractor with a pretensioner function is furthermore provided with a force limiting device by means of which a force-limited, seat-belt strap extraction is facilitated. The force on the passenger during the force-limited forward displacement is determined by the design of the force limiting device, whereby various force limiting sequences can be adjusted by combining several force limiting elements.
The disadvantage with systems mentioned above, however, is that because of the successive opposite rotating movements of the belt shaft during the belt tightening process which produces belt retraction, and the force limitation which occurs during belt extension, the seat-belt strap force limitation can be obstructed by the still applied tensioning force exerted by the seat-belt pretensioner. This disadvantageous force limitation impediment can thereby result in a short-term peak at the start of the force limiting process.
A seat-belt pretensioner of the above-referenced type is known from WO 2009/118088 in which a locking element is provided to couple the pretensioner drive wheel to the belt shaft and in which the locking element can be detached by turning of the belt shaft in the direction of the seat-belt strap extraction after the tensioning movement has been completed. The pretensioner drive wheel and the belt shaft are coupled with each other by means of a ramp contour which forces an axial movement of the tension drive wheel with respect to the belt shaft. The locking element is formed, for example, by a spring disk which is moved into a ready position by the activation of the tensioning movement in that it engages with spring arms in a fixed-housing gear. By means of the developing force-limited seat-belt strap extraction movement, the locking element fixed in the fixed-housing gear in the direction of the belt extraction executes a release movement and thereby enables the axial movement of the pretensioner drive wheel with respect to the belt shaft, whereupon the pretensioner drive wheel and the belt shaft disengage.
A disadvantage of the solution described above is that the locking element must first be brought to a ready position by the tensioning movement in which it is fixed in the seat-belt strap extraction direction with respect to the housing. Furthermore, the locking element couples the pretensioner drive wheel to the belt shaft by exerting an axial force and must therefore have a certain form stability in order to absorb the impacting forces.
The object of the invention is to create a seat-belt pretensioner in which the disadvantages mentioned above are avoided.